The two concepts are winding the thread around the plastic so the correct amount goes into the melt zone so it will exit at the correct nozzle extrusion speed the other is some thoughts on a coaxial nozzle that would work much like a wire insulation machine but much slower.

Interestingly I saw a machine that is used to make flux cored solder a couple of months back. It is a solid solder extruder that presses a billet of metal through an annular nozzle. The centre is continuously filled with molten flux as the solder extrudes. They had it easy because the diameter on exit was about 7-10 mm. It was then fed through two progressive wire drawing machines that stretched and thinned the cored solder to sizes from 2.4 to 0.5 mm. The interesting thing to note was that the extruding solder did not try and push into the flux feeder, it rather drew the flux down into the hollow tube with gravity and created partial vacuum.

Have a look and discuss if interested.

It is a concept I am personally unlikely to test but it might be just the thing someone needs.

Kalle
--
Lahti, Finland
The only stable form of government is Open Source Government. - Kalle Pihlajasaari 2013

Approx 25 years ago I made a co-fibre extruder, it premade the fillament though, the 3d printer needed a flying cutter to cut the fibre under the nozzle,
I had to build the extruder, then build the 3d printer, in those days stepper motor boards and software were not off the shelf.
it was a very interesting project.